1. Field of the Invention
The present invention relates generally to a method and apparatus for multi-phase fluid flow measurement based on the Coriolis principle. The apparatus according to the invention is particularly useful for gas-water-oil mixtures being produced from underground reservoirs and the like. Other areas of application for such a method and apparatus would include chemical, food processing, pharmaceutical, and other industries.
2. Description of the Prior Art
Efforts to measure the total mass flow as well as the flow rates of the components of the multi-phase fluid mixtures such as a gas-water-oil mixtures which are typically produced from oil and gas wells have resulted in the development of several types of flow measuring systems.
It is known to determine the mass flow rate of individual components of the multi-phase medium by method comprising preliminary separating a mixture into components and measuring the mass flow rate of each component separately, such as described in the U.S. Pat. No. 4,852,395 by Kolpak. This method is rather complicated and inaccurate since the accuracy of measurement depends on the quality of separation of the mixture. It is also known to measure the flow rate of individual components of two-component flows wherein volumetric and mass flow rates of a two-component flow are measured by using volumetric and mass flow rate sensors. Such sensors are mounted serially along the flow path. Signals proportional to the values of the measured flow rates are fed to a computer which solves a set of two equations with respect to variables representing volumetric and mass flow rates of individual components. In this case, the accuracy of measurements depends on fluctuations of the flow structure, variations of the pattern of velocity and pressure fields which are inevitable in different points of the pipeline. These devices have satisfactory performance as long as the gas/liquid ratio is not greater than 1:2. In practice, however, this ratio can go up to 25:1 or more.
In the past few years, a new type of flow meters had been under development based on the Coriolis principle. This type flow meter has the advantage of being simple and reliable and also lacks mechanical components influencing the structure of the measured flow. The accuracy of measurements does not depend for these devices on such physical characteristics of flow as viscosity, density, and temperature. The sensing element of such a flow meter is a hollow flow containing pendulum which is subjected to vibrations as the flow goes by. The Coriolis force is produced when the radius of the mass traveling in angular motion inside such pendulum is changed. The changing radius causes a corresponding change in angular acceleration which produces a net force tangent to the direction of rotation. The Coriolis force is effecting, depending on the way the hollow pendulum is fixed, the amount of energy losses in the vibrating system consisting of the pendulum itself and the measured flow.
There are two fundamental ways to fix the oscillating pendulum in place. In the first, cantilevered way, only one end of the oscillating pendulum is fixed in place while the other one is free to move. The flow rate is measured indirectly by the amount of additional energy necessary to compensate for the energy losses and maintain the oscillations at the same level. The second way is to fix both ends of the oscillating pendulum in place. In that case, the total energy loss of the whole system remains constant regardless of the form of the pendulum since the Coriolis forces on both sides of the pendulum work against each other. The flow information is derived from the changes in the vibration parameters and shape of the pendulum under the influence of Coriolis forces which tend to apply a "twisting" motion to the pendulum. These are the systems of direct measurement.
The flow meters based on the principle of direct measurement have been under development for the last few years. A wide variety of U-shaped and .OMEGA.-shaped symmetric hollow oscillating pendulums are known to be developed by various companies working in this area. All of the known flow meters, however, have a similar drawback. Since the changes in the vibration parameters and pendulum shape under the influence of Coriolis forces are measured using the constant frequency other than the natural resonant frequency of the system, and since the Coriolis forces are naturally very small, while, at the same time, the stiffness of the hollow conduit is quite high, the absolute values of pendulum deviations are quite low and difficult to measure. Due to this factor, these devices work well with a single-phase flow, while in the case of a multi-phase flow the accuracy of the flow measurement is not acceptable due to the high level of hydrodynamic disturbances. Examples of such systems can be found in U.S. Pat. Nos. 5,429,002 by Colman; 5,287,754 by Kazakis; 5,044,207 by Atkinson, and 5,024,104 by Dames.
The U.S. Pat. No. 5,604,316 by Alonso describes a multiple phase Coriolis mass flow meter. The device includes a rotating member made up of a plurality of flow conduits arranged symmetrically around a shaft and driven by a motor. The conduits are attached on both ends to the shaft with one end closer to the shaft central line than the other end. The Coriolis force is imparted to the conduits while bending beam load cells are attached to measure the force. That device is rather complicated, requires a motor or an engine and an additional housing around the outlet of the conduits.
A gas/liquid flow measurement system using a Coriolis based flow meter is described in the U.S. Pat. No. 5,0290,482 by Liu. The method according to the invention uses a Coriolis based mass flow meter. Flow streams of known mass flow rate and phase distribution are directed from the meter and correlation factors are obtained using an apparent mass flow rate output and an apparent density output from the Coriolis meter. The true mass flow rate and phase distribution of unknown flow streams can then be determined. The limitation of this device is that it works well only in high gas/liquid ratio flows such as a wet stream.
A flow-meter for determining a flow rate of liquid or slurry flowing in the pipeline is described in the U.S. Pat. No. 4,856,347 by Johnson. The meter includes a tube-like J-bend member which is connected to the pipeline by a non-restricting, ring-type universal joint. Although useful for a multi-phase flow measurements, this device comprises an open-ended conduit, which makes it practical only in specific applications.
Another Coriolis type fluid flow meter and a multi-phase fluid flow measurement system is described in the U.S. Pat. Nos. 5,090,253 and 5,259,250 by Kolpak. A complicated measurement system includes a volumetric meter such as a Coriolis flow meter, a densimeter, and another meter which measures the fraction of one liquid in the two liquid mixture. A Coriolis type flow meter has separate bundles of tubes which are interconnected with each other to vibrate laterally with respect to the direction of flow through the tubes by vibrating mechanism and which vibrations are sensed by spaced apart sensor devices for determining the vibrational characteristics and related mass flow rate and mass density of fluid flowing through the tube bundles. In one embodiment, the inlet and the outlet manifold are arranged for parallel lateral flow of fluid to and from the manifolds and the respective sets of flow tubes are spaced apart along the manifolds and arranged in alternating sets interconnected by tie rods at the respective vibrator mechanisms and sensors. As noted in the description of the invention, this device works satisfactorily only if the gas to liquid ratio is not more than about 1:5 given that the diameter of the flow conduit is about 1/4 of an inch. In reality, this ratio may be much higher and therefore, this device has limited utility. Another problem is that the arrangement of parallel conduits is done in such a way that the stiffness of the overall device is very high. This affects the accuracy of the measurements in a negative way by further reducing the absolute values of the oscillations to be measured.
A two-phase Coriolis flow meter is described in my U.S. Pat. No. 4,096,745 which is incorporated herein by reference. Although a cantilevered design is described with only one end of the oscillating pendulum fixed in place, this patent contains a description of several important principles for the design of the Coriolis based multi-phase flow meters which are important to keep in mind designing any type of such a system. Here is a brief summary of these principles:
The natural frequency of oscillations of the pendulum filled with the medium should be at least ten times smaller then the fundamental frequency of the medium itself which can be closely approximated to be equal to the ratio of the speed of sound travelling in the medium over the size of the pendulum in the plane of its oscillations (pendulum diameter in case of a round shape). This relationship may also be represented by the following formula: EQU f.delta..ltoreq.30, PA1 f--is the frequency of pendulum oscillations measured in 1/c, and .delta.--is the size of the pendulum in the plane of oscillations measured in cm. For a round pendulum it is the inside diameter of the pendulum. In that case, the oscillations amplitude of the pendulum would not depend in any appreciable way on the changes in energy losses introduced by the flowing medium and so the error of flow measurements as was determined by us experimentally would not exceed about 5% for the multi-phase medium and respectively 0.2% for the single-phase medium. These low levels of measurement error are practically acceptable by the industry; PA1 The pendulum diameter should be designed in such a way that the minimum speed of the flowing gas/liquid medium is sufficiently high, and hence, the Froud number is sufficiently high as well to maintain a type of a multi-phase flow known as a self-similar flow. In that case, the ratio of flowing gas speed to the flowing liquid speed is constant and the relationship between the actual volumetric gas content and the flow rate gas content is linear. It was demonstrated empirically that in case of ascending and horizontal flows, the value of the Froud number should be higher than 4 in order to satisfy this requirement. In case of descending flow conditions and depending on the nature of the gas/liquid flow, self-similar flow conditions occur at higher Froud numbers; PA1 Integration time for calculating the flow parameters should be chosen in such a way that even under the highly unstable high gas/liquid ratio flow conditions, the accuracy of measurements should not fall below the required minimum. Highly unstable flow conditions occur when large quantities of gas coalesce together to form a large bubble or "plug" along the flow pathway and a plurality of these bubbles of various sizes flow in the liquid medium. As can be appreciated by those skilled in the art, such bubbles represent a major disruptions in the continuity of flow. Should the integration time be chosen small, under these conditions the accuracy of flow measurements would be significantly decreased. It was empirically determined that a 100 second integration time interval would yield a fairly high accuracy of not worse then 1.5% under any flow conditions including those described above. To get an even higher accuracy of, for example, not worse then 0.5%, a 1000 second integration time interval is required which is still not unreasonable for a typical oil well. In addition, an optional flow conditioner may be employed prior to the entrance to said device to more evenly mix the flow and allow for increased accuracy of flow measurement, especially those flows experiencing large gas bubbles or plugs. PA1 In case of high absolute flows and low gas/liquid ratios, a zone of low pressure may form inside the device housing which may cause periodic interruptions in the flow measurements; PA1 In case of descending flows, the self-similar flow conditions require the Froud number to be greater then 50 which, in turn, cause the pressure drops across the oscillating pendulum to be quite high as well. This limits the utility of the flow meters for some specific liquids; PA1 The open ended pendulum causes additional disturbances in the flow conditions which reduce the accuracy of flow measurements; and finally PA1 Design of the flow meter capable of working with high pressure drop across the pendulum demands the use of heavy metallic components which makes the device heavy and more costly.
where
At the same time, a cantilevered type design described in my above referenced patent, has the following limitations typical for all flow meters of that type:
Therefore, the basic technical controversy is as follows: it is desirable to switch from a cantilevered type flow meter with energy loss compensation method of measurement to the one in which both ends of the oscillating pendulum are fixed. However, direct measurement method typically used for these preferred type flow meters does not yield sufficiently high measurement accuracy in case of a multi-phase flow conditions. The need therefore exists for a Coriolis based flow meter where both ends of the pendulum are fixed in place which is capable of accurate and reliable measurements of individual multi-phase flow components, such as gas and liquids with a wide range of gas/liquid ratios.